With the rapid growth of wireless telecommunication systems, system providers are challenged to provide quality radio frequency (RF) signals with maximum coverage. In a wireless telecommunications system, a mobile phone needs to be calibrated to provide the power necessary to reach the base station. On the other hand, in order to conserve battery life, the power transmitted should not be more than what is needed. The mobile phone must be able to tune the output power for an optimum connection. Also, the base station and mobile phone must not exceed the maximum allowable power transmitted according to government standards and regulations. Therefore, assuring that the transmitted power does not exceed the allowable limit, which is traced to a known standard, is a primary concern. Thus, accurate power measurements are important for maintaining a high quality connection in modern telecommunication systems.
As with many other types of signals, an RF signal can be made up of a sequence of pulses. In the case of a pulsed RF power signal, the pulses have a leading rising edge and a trailing falling edge. The power envelope of the RF signal is, in some cases, determined by the RF signal's modulation type.
Standard peak or sampling RF power meters include trigger circuitry which has to be capable of detecting the leading and trailing edges on the incoming power envelope. The detection of leading and trailing edges is then used in a data acquisition circuit in order to control the storage of signal samples in memory for later processing and measurement extraction. In order to try to detect the edges of the pulse, it is known to use a trigger level, or threshold, to detect whether the signal has crossed the threshold.
However, RF power envelopes can be very noisy and the power pulse may be heavily modulated, so that the modulation envelope of the pulse may cross over the pre-programmed trigger threshold at times other than the desired edges of the pulse. One well known method of coping with noisy signals is to apply hysteresis and trigger validation to the detection measurement.
FIG. 1 (a) shows a typical well behaved (but noisy) detected RF power signal 13 having a leading rising edge rather masked by the noise spikes. The desire is to find the leading edge i.e. the point at which the detected RF power signal 13 crosses a trigger threshold 10. As can be seen, the noise spike at point X exceeds the trigger threshold 10, but the detected RF power signal 13 then immediately drops back below the trigger level 10. The first point at which the detected RF power signal 13 is wholly above the trigger threshold 10 is point Y. However, if the noise were to be filtered out then the actual trigger point would be at point Z. Therefore, to get a more accurate trigger, hysteresis is introduced, as shown in FIG. 1 (b). In this case, instead of having a single trigger threshold, two thresholds are provided, one on each side of the actual desired trigger threshold: an upper hysteresis threshold 11 and a lower hysteresis threshold 12.
In order for the trigger circuitry to detect a rising edge, the detected RF power signal 13 must rise above the upper hysteresis threshold 11 and not fall back below the lower hysteresis threshold 12. With this technique, point Z is the first point at which the detected RF power signal 13 has risen above the upper hysteresis level 11, but not fallen again below the lower hysteresis threshold 12. Even though the detected RF power signal 13 may fall below the actual trigger level 10, after rising above the upper hysteresis threshold 11, it does not fall below the lower hysteresis level 12 and therefore the rising edge is detected as being at point Z.
A trigger circuit can be made to trigger on very noisy signals by increasing the distance between the hysteresis thresholds. If the detected RF power signal 13 was pulsed, then there would always come a time when the signal did drop below the lower hysteresis threshold 12. For this reason the concept of validation is used, as shown in FIG. 2. In this case, qualification time 14 is set such that the signal 15 must remain above the lower hysteresis threshold 18 for the qualification time 14, in order to be considered a rising edge. The qualification time 14 must be carefully chosen to be large enough to reject noise, but small enough not to reject genuine pulses.
The qualification time 14 works for both rising and falling edges. A signal 15 that falls below the lower hysteresis threshold 18, must stay below the upper hysteresis threshold 16 for duration of the qualification time 14 for a falling edge to be considered to have been detected and the signal therefore to be “low”. Once low, the signal 15 must then rise above the upper hysteresis level 16 and stay above the lower hysteresis level 18 for a rising edge to be considered to have been detected and the signal therefore to be in a “high” state. Validated low-to-high and high-to-low transitions constitute rising and falling edges, respectively. The method described so far is well known and in widespread use in oscilloscopes, as well as power meters.
A problem arises when the pulsed signal is a burst of digital modulation. Digital modulation, such as OFDM or 64QAM, can cause the signal envelope to cross over the trigger thresholds throughout the on-time of the signal, or pulse to be detected. Hysteresis and validation are needed for noise rejection, but, for example in the case of the waveform shown in FIG. 2, a qualification time that works well for noise rejection could cause a whole pulse to be missed, because the signal 15 always drops below the lower hysteresis threshold 18 within the qualification time 14 of every positive transition.
If the qualification time 14 is reduced to allow the front edge of the pulse to be detected then, in the example shown in FIG. 2, the modulation will meet the validation criteria, resulting in five small pulses being detected instead of one larger one. The known technique of trigger hold-off can be used to prevent problems arising from the multi-triggering by suppressing triggers for a time after the first detection. This also has the limitation that, when presented with pulses of non-deterministic width or interval, as can be present in current or proposed wireless LAN signals, it cannot provide a stable trigger. Thus, the known techniques cannot distinguish between distinct pulses, or a single pulse modulated at a frequency around twice the pulse width, or even noise spikes at a similar frequency.
The present invention therefore seeks to provide a method and apparatus for detecting leading pulse edges, especially, though not exclusively for detecting leading edges of heavily modulated pulses and/or pulses in a noisy signal, which overcomes, or at least reduces the above-mentioned problems of the prior art.